1. Field of the Invention
The present invention relates generally to the prevention of mid-plane delamination failures in laminated composite flexures. In particular, the present invention relates to a method and apparatus for increasing the strength and fatigue life of helicopter main rotor yokes constructed of laminated composite materials.
2. Description of Related Art
Helicopter main rotor yokes combine to form main rotor hubs and provide means to connect the main rotor blades to the main rotor mast, which provides the necessary drive torque. These main rotor yokes are typically elongated members having center sections adapted for connection to the rotor mast, and outwardly extending arm sections. As is well known, the main rotor blades of helicopters exert high centrifugal forces on the rotor yokes of the main rotor hub. In addition to the high centrifugal forces, the main rotor yokes are subjected to a variety of other static and dynamic forces and motions, including feathering, flapping, and lead-lag forces and motions. In general, feathering forces and motions are torsional pitch forces and motions generated by the variable pitch of the rotor blades; flapping forces and motions are bending forces and motions generated by the rotor blades out of the plane of rotation of the rotor blades; and lead-lag forces and motions are bending forces and motions generated by the rotor blades in the plane of rotation of the rotor blades.
Because rotor yokes are subjected to such a large variety of tension and bending forces and motions, they must be both strong and flexible. Often this can only be achieved by the addition of moving parts and linkages. As such, conventional main rotor yokes and rotor hubs have to include complicated systems of bearings and supports to accommodate these forces. These are usually complex systems requiring a great deal of maintenance and service in the form of lubrication and inspection. This leads to high costs in the form of increased labor, increased materials, reduced operating time, and reduced service life. However, in recent years, these conventional rotor yokes, which require complex bearing and lubrication systems, have given way to xe2x80x9cbearinglessxe2x80x9d rotor yokes made from composite materials, which do not require any bearings or lubrication systems.
Composite bearingless rotor yokes are specifically designed to compensate for the centrifugal, feathering, flapping, lead-lag, and other static and dynamic forces and motions exerted upon the rotor yokes by the rotor blades. Because these bearingless rotor yokes are composite materials, they provide all of the benefits generally associated with composite flexures: high strength-to-weight ratios, reduced weight, and reduced maintenance; without the need for costly linkages, bearings and lubrication systems. The lengths, thicknesses, and cross-sectional profiles of these composite rotor yokes can be varied to produce very specific mechanical strengths and properties. However, once the appropriate configuration and dimensions of such a rotor yoke have been determined for a particular application, there is little or no room for additional structural enhancements.
Although components manufactured from laminated composite materials offer the benefits listed above, it is commonly known that laminated composite flexures, particularly laminated composites having exposed edges, are susceptible to failure in the form of mid-plane delamination. Mid-plane delamination initiates at the exposed edges of the laminate due to edge effect concentrations and progresses inward toward the center. It is also commonly known that inter-laminar shear stresses are concentrated at the edges of laminated composite flexures. Bearingless composite rotor yokes are no exception to these rules. The cyclic bending loads and motions, both in-plane and out-of-plane, exerted upon composite rotor yokes can lead to mid-plane delamination fatigue failure.
Although the use of composite materials in constructing helicopter main rotor yokes represents a significant advancement in the art, the problem of mid-plane delamination has not been adequately resolved.
It is an object of the present invention to provide an edge member for protecting laminated composite flexures having exposed edges by reducing or eliminating failure in the form of mid-plane delamination.
It is another object of the present invention to provide a method of reducing or eliminating failure in the form of mid-plane delamination in laminated composite flexures having exposed edges.
It is another object of the present invention to provide a helicopter having a bearingless main rotor yoke made of composite materials for which fatigue failure in the form of mid-plane delamination is reduced or eliminated.
It is another object of the present invention to provide a bearingless main rotor yoke for a helicopter, the bearingless main rotor yoke being made of composite materials for which fatigue failure in the form of mid-plane delamination is reduced or eliminated.
It is another object of the present invention to provide a method of extending the service life of a bearingless helicopter main rotor yoke made of laminated composite materials, the method including the step of adding composite edge members onto the edge of the bearingless helicopter main rotor yoke.
The above objects are achieved by providing an edge member having an innermost, an outermost, and internal layers or plies. The internal plies of the edge member conform to an exposed edge surface of the composite flexure. These internal plies have a height approximately equal to the height of the edge surface of the composite flexure. The innermost and outermost plies of the edge member also conform to the edge surface of the composite flexure. However, the innermost and outermost are larger than the internal plies so that the innermost and outermost plies can be overlapped onto and attached to the uppermost and lowermost plies of the composite flexure.
The present invention provides significant advantages. Because inter-laminar shear stresses generated within the composite flexure peak at or near the edge of the composite flexure, the edge member of the present invention can be configured to isolate the peak shear stresses. Because only the innermost and outermost plies of the edge member of the present invention are overlapped onto the composite flexure, the thickness of the composite flexure is not significantly increased. The edge member of the present invention protects the composite flexure from random impact damage, as from handling. Because the edge member of the present invention places a watertight seal on the exposed edges of the composite flexure, moisture uptake by the composite flexure in high humidity environments is greatly reduced. These and other objects and advantages of the present invention will be apparent in the following detailed description when read in conjunction with the accompanying drawings.